1. Field of the Invention
The present invention relates to a correlation detecting device for detecting a specified code contained in a received signal and its method, and more particularly to a preferred correlation detecting device and its method suitable for use when setting the timing using a specified code in a wireless LAN wirelessly connecting a plurality of terminals.
2. Description of the Related Art
With an increase in ability of a computer, a LAN (Local Area Network) connecting a plurality of computers for the purpose of sharing files and data, and transfer of data and an electronic mail are prevalent. In the conventional LAN, each computer is connected together by wiring with an optical fiber, a coaxial cable, or a twisted pair cable.
This wired LAN, however, needs a work for connection. This makes it difficult to build up a LAN, and wiring becomes complicated in the cable LAN. Then, a wireless LAN has drawn attention as a system for releasing a user from wiring of the conventional wired LAN system.
As a wireless LAN, such a method as performing data communication by CDMA (Code Division Multiple Access) using spread spectrum, has been proposed. In the CDMA method, communication data is multiplied by a PN code (Pseudo Noise Code), and the spectrum of the communication data is spread. Thus spectrum spread and transferred data is multiplied by the same PN code as that of the transmitting side to be demodulated. The CDMA method is characterized by high secrete ability and high interference resistant ability.
With a progress in multimedia information, large data in amount such as image data and sound data is more often handled in these days. Accordingly, a wireless LAN is also required to speed up the transfer rate so as to transfer the large amount of data such as the image data and the sound data. However, if data transfer is performed, for example, at a high rate of about 30 Mbps, in the spread spectrum modulation, the bandwidth of more than 300 MHz will be required. This wide bandwidth cannot be ensured in the current frequency assignment, and it is difficult to do communication while ensuring this wide bandwidth.
The spread spectrum needs a synchronization capture time for adjusting the phase of the code of the transferred data to the phase of the code generated by a receiver for demodulation. Therefore, in the spread spectrum, a bit string for synchronization is inserted into each packet, for synchronization at a high speed, and this bit string for synchronization causes a problem of increasing the bits other than those of the effective data.
Then, the present applicant proposes a technique of transferring data in the OFDM (Orthogonal Frequency Division Multiplexing) method, performing data communication by the TDMA (Time Division Multiple Access) with one frame fixed as the unit, sending the Maximum Length Code at the head of one frame, setting the sending and receiving timing with the Maximum Length Code as a reference, and instructing the sending and receiving time of each wireless communication terminal according to the control information from a wireless communication control terminal. The OFDM method can increase the transfer rate, and assure the accurate demodulation even if there is a jitter. Since the sending and receiving timing is established, with the Maximum Length Code at the head of a frame made as a reference, it is possible to demodulate only a necessary symbol within a frame so as to reproduce the data by using this time information in reception.
When setting the timing upon receipt of the Maximum Length Code in this way, it is necessary to detect the Maximum Length Code from a received signal. As a circuit for detecting the Maximum Length Code like this, a correlation detecting circuit using a matched filter may be considered.
FIG. 23 shows an example of the correlation detecting circuit using a matched filter. In FIG. 23, a received signal from an input terminal 151 is supplied to a matched filter 152. The matched filter 152, that is a kind of FIR filter, comprises delay circuits 161-1, 161-2, 161-3, . . . , multipliers 162-1, 162-2, 162-3, and an adder 163 as shown FIG. 24. The coefficient of the multipliers 162-1, 162-2, 162-3, . . . is set at 1 or (xe2x88x921) depending on the detected code. When a correlation between the code set to the coefficient of the multipliers 162-1, 162-2, 162-3, . . . and the received code is strong, the output level of the adder 163 becomes larger.
The matched filter 152 detects a correlation between the received code and the code set to the filter. The output of the matched filter 152 is supplied to a comparator 153. The comparator 153 compares the output of the matched filter 152 with a predetermined threshold TH.
Upon receipt of the Maximum Length Code, the output of the matched filter 152 becomes larger than the threshold TH. When the output of the matched filter 152 becomes larger than the threshold TH, a detected output appears from an output terminal 154.
In this system, however, each signal from a plurality of wireless communication terminals is time-division multiplexed within a frame to be, transferred and the level of each signal received by each wireless communication terminal and a wireless communication control terminal differs. Therefore, even if the received signal of the Maximum Length Code appears from the matched filter 152 which has received the Maximum Length Code, this signal output hides in the other signal component, and the signal of the Maximum Length Code cannot be detected easily.
Specifically, AGC (Automatic Gain Control) works on a received signal so as to control the level of the received signal at a constant level. When a signal of excess amplitude is input just before the signal of the Maximum Length Code in this condition, the AGC works so to reduce the gain. When the Maximum Length Code is received in a state of the reduced gain, the detected level of a correlation signal of the Maximum Length Code becomes smaller, and the correlation signal hides in the strong received signal, so that it cannot be detected. Even if only the vicinity of receiving the Maximum Length Code is extracted, the detected level of the correlation signal of the Maximum Length Code becomes less than the threshold, and so the signal of the Maximum Length code cannot be detected.
As described above, when a plurality of signals having various signal levels are sent in time division, a design of comparing the output of the matched filter with the threshold cannot assure the detection of the signal of the Maximum Length Code.
In order to solve the above problem, the present applicant previously proposed a correlation detecting circuit capable of detecting a correlation of the code of the received Maximum Length Code satisfactorily. FIG. 25 shows the structure of the correlation detecting circuit proposed previously, in which a received signal is supplied to an input terminal 171. The received signal is supplied to a matched filter 172 as well as to an average amplitude detecting circuit 173. The output of the matched filter 172 and the output of the average amplitude detecting circuit 173 are supplied to a divider 174. The divider 174 divides the output level Sa of the matched filter 172 by the output level Sb of the average amplitude detecting circuit 173. The output of the divider 174 is supplied to a comparator 175. The comparator 175 is provided with a predetermined threshold TH. The comparator 175 compares the output of the divider 174 with the threshold TH. The output of the comparator 175 is supplied to an output terminal 176 as the detected output of a correlation.
Referring to the correlation detecting circuit as shown in FIG. 25, when receiving the Maximum Length Code, the output level Sa of the matched filter 172 becomes larger. Therefore, the output of the divider 174 becomes larger than the predetermined threshold TH: Sa/Sb greater than TH; and a correlation detecting signal appears from the comparator 175. The correlation detecting signal is supplied from the output terminal 176.
When the amplitude of an input signal is great, the output level Sa of the matched filter 172 also becomes larger. However, when the amplitude of an input signal is great, not only the output level Sa of the matched filter 172 but also the output level Sb of the average amplitude detecting circuit 173 becomes larger. Therefore, the value (Sa/Sb) obtained by dividing the output level Sa of the matched filter 172 by the output level Sb of the average amplitude detecting circuit 173 hardly changes, and the output of the divider 174 is smaller than the predetermined threshold TH: Sa/Sb less than TH. Therefore, no correlation detecting signal appears from the comparator 175.
Thus, when the output of the matched filter 172 is normalized by the amplitude level supplied from the average amplitude detecting circuit 173, the detected level does not become larger even if the level of the input signal becomes larger, and so the receipt of the Maximum Length Code can be detected correctly.
The correlation detecting circuit previously proposed, however, is defective in that its circuitry is complicated because of using a divider. To be specific, a divider needs larger size as compared with the other operational circuits such as a multiplier, because arithmetic operation of many digits at decimal level is required in the case of a divider performing an accurate division.
Accordingly, the present invention aims to detect the code sequence assuredly in a simple circuit without a divider, even if a plurality of signals of different amplitudes is time-division multiplexed.
A correlation detecting device according to the present invention comprises matched filtering means for detecting a predetermined code from a received signal, average amplitude detecting means for detecting the average amplitude of the received signal, multiplying means for multiplying an output of said average amplitude detecting means by a threshold, and comparing means for supplying a correlation detecting signal by comparison between the output of said matched filtering means and the output of said multiplying means.
A correlation detecting method according to the present invention comprises the steps of detecting a predetermined code from a received signal by a matched filter, detecting the average amplitude of the received signal, before multiplying the detected signal of the average amplitude by a threshold, and comparing the multiplied signal with the output of the matched filter to obtain a correlation detecting signal based on the above comparison.
According to the present invention, the output value of a matched filter is compared with the value obtained by multiplying the average amplitude by the threshold, so as to detect the Maximum Length Code from a received signal. Since the output of the matched filter is in proportion to the level of an input signal, the output becomes larger not only in the case of receiving the Maximum Length Code but also in the case of receiving a signal having a great level. When the level of the received signal is large, the output level of the matched filter becomes larger. At this moment, the average amplitude of the received signal becomes also large and the value obtained by multiplying the average amplitude by the threshold becomes large also. Therefore, when the output level of the matched filter is compared with the value obtained by multiplying the average amplitude by the threshold, the ratio of each level makes little change. On the contrary, the output level of the matched filter when receiving the Maximum Length Code is substantially kept at constant independently of the level of an input signal. Accordingly, by comparing between the output value of the matched filter and the value obtained by multiplying the average amplitude by the threshold to detect the Maximum Length Code from a received signal, the detection of the Maximum Length Code from the received signal is assured.